In the offshore production of oil, gas, and other production fluids, floating vessels are frequently used to transport the production fluids to onshore consuming markets. The production fluids are produced from an offshore structure which is firmly anchored to the sea floor with pilings. To convey production fluids from a subsea well to the water surface, the offshore structure supports a fluid-carrying system called a riser. A flowline connected to the upper end of the riser conveys the production fluids to a storage tanker which may be permanently moored to the offshore structure. Shuttle tankers offload the production fluids from the storage tanker for transportation to an onshore market.
In a water environment, a mooring system must be sufficiently flexible to accommodate movement of the vessel relative to the offshore structure. As the vessel is acted upon by loading forces induced by wind, waves, ice, and ocean currents, the vessel will roll, pitch and heave. In addition, the vessel will yaw about its mooring point as the direction of the loading forces vary. As the vessel moves, it will impart dynamic forces which tend to damage a rigid mooring structure and the attached offshore structure. Therefore, a rigid mooring system is undesirable in a water environment because it is unable to accommodate relative movement between the vessel and the offshore structure.
To provide a flexible mooring system in a water environment, nylon hawsers are used to moor a vessel to an offshore structure. Because nylon is an elastic material, nylon hawsers dampen the dynamic forces which are induced by movement of the vessel.
Although nylon hawsers are sufficiently strong to moor a storage vessel to an offshore structure in a water environment, nylon hawsers cannot safely be used in a cryogenic environment such as the Artic. During the Artic winter, nylon loses its resiliency and becomes brittle. This brittleness reduces the breaking strength of a nylon hawser which may lead to failure of the hawser. Weakened nylon hawsers are particularly susceptible to failure as moving pack ice acts against a moored storage vessel. Moving pack ice containing ice ridges up to thirty feet in height will exert enormous forces against a moored vessel.
Although nylon hawsers cannot safely moor a storage vessel throughout the year in an Artic environment, rigid mooring chains are sufficiently strong to withstand the forces induced by moving pack ice. However, rigid mooring chains are not suitable in a water environment because they are not sufficiently elastic to accommodate movement of the storage vessel. As the vessel moves toward the structure, the mooring chains can become slack. If loading forces urge the vessel away from the structure, the vessel can gain sufficient momentum to impart a large impact force to the mooring chains and connected structure when the vessel reaches the excursion length of the chains. In such event, the structure or chains may be damaged.
While nylon hawsers can moor a vessel in a water environment and rigid mooring chains can be substituted for nylon hawsers during the winter months, valuable production time will be lost when the fluid-carrying system is shut down to convert from nylon hawsers to the mooring chains. There is, therefore, a need for a mooring system that can accommodate loading forces acting on a vessel due to wind, waves, ice, or ocean currents. Further, there is a need for a mooring system that can be used in a water environment and in an ice bound environment.